My ex girlfriend and I broke up two years ago in very good terms and we've been friendly since then and we've only had contact when we hanged out with some mutual friends.

Recently I plated a friend of my Ex's best friend, which is also a friend of mine. For the sake of simplicity, let's call my new plate E and my ex's best friend Y.
E, Y and I have been hanging out together with the mutual friends I have with my ex. What happens is that E is a bit uncomfortable about meeting my Ex which normally hangs out with this group of friends I've mentioned. I don't really know why because I haven't showed any concern about this possibility but whatever.
My ex knows I'm fucking E and she also knows that E doesn't feel comfortable meeting her because someone told her probably. It turns out she's a bit pissed because she thinks E is trying to steal her group of friends from her. Basically the thing has snowballed big time.

My current stance is not picking sides and letting the problem solve itself but when shit hits the fan (when they finally meet in some social event or something) I'm not sure how to act. What attitude will you guys have in a situation like this?

PD: English is not my first language by the way.